Impregnation of insulating materials



IMPREGNATION 0F INSULATING MATERIALS Filed March 17, 1930 I ATUR/VEY Patented Dec. 31, 1935 UNITED STATES IMPREGNATION OF INSULATING MATERIALS Girard '1. Kohman, West Orange, N. 3., assignor to Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application March 17, 1930, Serial No. 436,550 4 Claims. (01. 91-70) This invention relates to insulating materials and more particularly to a process for impregnating insulating materials to improve their electrical characteristics.

When a cellulose material such as paper is used for insulating purposes as for example in condensers or in electric cables, it is customary to impregnate the paper with a suitable insulating compound to improve its electrical characteristics. Heretofore it has been a common practice to remove the moisture from the paper immediately before impregnation as for example by subjecting the paper to a high temperature. In certain instances, however, it was found that the dielectric strength of paper treated in this manner was not sufliciently high.

The object of the present invention is to improve the electrical characteristics of cellulose insulating materials such as paper.

In accordance with the present invention this object is attained by impregnating such materials with an insulated compound composed in substantial part of suitable polar molecules while the cellulose molecules of the paper are in an active condition.

For a clearer understanding of this invention reference may be had to the following detailed description and accompanying drawing showing a condenser which is a piece of apparatus to which this invention is particularly applicable.

As shown by the drawing this condenser consists of two sheets l and II of metal foil separated from each other by sheets I! and [3 of a suitable insulating material such as paper.

' Metallic terminals l4 and I5 secured to sheets l0 and II respectively serve to make-external electrical connection. In the preferred embodiment of this invention these sheets are wound and the unit thus formed is subjected to a preliminary pressing to obtain the desired shape after which the unit is subjected to vapors such as the vapors of water or alcohol at temperatures which may vary between 50 and 150 C. and it is then immediately placed in a suitable vacuum tank containing an impregnating compound which is maintained at substantially the same temperature as the vapors and which consists in a substantial part of polar molecules. Chlorinated naphthalene commonly known as halowax has been found suitable and is preferably employed as the impregnating compound in this process. After the unit is submerged in the impregnating compound the vapor molecules are removed and the active forces which result attract the polar molecules thus producing a bond between the cellulose molecules and the impregnating compound. After impregnation the unit is removed from the tank and again pressed to remove the excess impregnating compound after which it is sealed in a suitable container 5 by any of the standard potting compounds. The theory as to how a firm bond is established between the cellulose molecules and the impregnating compound is believed to be as follows: The cellulose molecules of the pulp from which the paper is made contain several hydroxyl groups,

a few of which may be represented as follows for the purpose of illustration:

1 H-c-oH .1110

1 HCOH .J "H10 (11) 1 HCOH -.Hz0

groups are near one another;

or as at (c) where the hydroxyl groups are farthest away from one another and form a part of a second molecule;

-(:)--OH OH-C If the paper, while the molecules are in form (1)) or (c), is placed, as is ordinarily the practice in active forces available a vacuum tankin the presence otthe impregnating compound, there would .be 'no residual. or hence the impregnating compound would be loosely held in the pores and on the surface of the paper. However, when the paper is exposed to the vapors at temperatures which may vary from C. to C. in accordance with the present invention, the cellulose molecules are reconverted to form (a) h nce, when the unit is placed ln'the vacuum tank in the presence of the impregnating compound and the absorbed vapors are removed, there are residual or active forces available which attract the polar molecules thus producing a firm bond between the paper and the impregnating compound.

In another embodiment of this invention the unit after the preliminary pressing is exposed to similar vapors at temperatures which may be between 50? C. and 150 C. and the vapor is then removedat relatively low temperatures after which the unit is immersed in a bath of a similar impregnating compound. After impregnation the unit is again pressed to remove the excess impregnating compound and then sealed in a container. In this embodiment when the unit is exposed to the vapors at these elevated temperatures the molecules which are ordinarily in form (12) or (c) are reconverted to form (a); When the vapor is then removed from the cellulose molecules at relativelylow temperatures the molecules are not sufliciently mobile for a rearrangement to take place, hence the residual forces remainactive. This for the purpose of illustration may be represented as:

- forces attract the polar molecules thus producheating at said temperaturts.

ing a flrm bond between the paper and the impregnating compound.

- While this invention has been described in connection with condensers, it is to be understood that it is equally applicable to other types'of 5 apparatus such as electric cables, transformers, etc.

What is claimed is:

l. The method of improving the electrical characteristics ofa cellulose insulating material which 10 consists in exposing said material to moisture at elevated temperatures within the range of 50? to 150 0., immediately immersing said material in a bath of chlorinated naphthalene, and then removing said moisture from said material at an 15 elevated temperature. I

2. The method of improving the insulating characteristics of a cellulose insulating material which consists in treating said material at a temperature of from 50 to 150 C. in the presence of 20 moisture until the cellulose molecules thereof are in an unsatisfied condition and then placing said material in a bath of chlorinated naphthalene.

3. The method of improving the insulating characteristics of a cellulose insulating material 25 which consists in treating said material at a temperature of from 50 to 150 C. in the presence of moisture until the cellulose molecules thereof are inan unsatisfied condition and then immediately impregnating said material with an insulating 30 compound a substantial part of which comprises polar molecules.

4. The method of improving the electrical characteristics of a paper insulating material which consists in exposing said material to moisture in 35 the form of alcohol vapors at temperatures of from 50 to 150 0., immediately immersing said material in a bath of chlorinated naphthalene, and then removing said moisture from said material by- 4.0 GERARD T. KOHMAN. 

